1. Field of the Invention
This invention relates to a system for preventing freshly printed sheets of a high speed printing device from offsetting.
2. Description of the Related Art
The American Heritage Dictionary defines "offset" as the unintentional or faulty transfer of ink not yet dry from a printer sheet to a surface, such as the next sheet that is laid over it. Offset is typically produced by the piling of sheets upon one another after imprinting. U.S. Pat. No. 2,110,219 describes a method for preventing offset in printing which a spray of a cloud of solvent material is applied to the freshly printed sheet. The solvent can be formed of lacquer gums, resin, cellulose esters or ethers which are soluble in alcohol, acetone or other solvents. A nozzle applies the cloud to the entire sheet of paper. A stack is formed of the treated sheets.
U.S. Pat. No. 3,556,519 describes a method and apparatus for stacking paper. A stream of compressed air is blown by nozzles below each successive sheet as the sheet advances from the printing unit to a collecting station. The air stream contains communicated solid material which is deposited to the underside of each advancing sheet. The above-described patents have the shortcoming of requiring the application of a solvent material to the printed sheets of paper.
U.S. Pat. No. 1,140,157 relates to a sheet piling mechanism for preventing freshly printing sheets from offsetting. Air is discharged intermittently towards the top freely dropping sheet. A nozzle moves in and out of the path of the piling sheets for temporarily supporting the uppermost sheet. The teachings of this patent have the disadvantage that the movement of the nozzle would undesirably interrupt the flow of sheets if used with a high speed printer.
Of possible general relevance are U.S. Pat. No. 4,132,400 to an apparatus for aligning a stack of sheets; No. 1,743,521 to a method of drying freshly printed sheets; and No. 1,465,385 to a sheet delivering mechanism.
It is desirable to provide an apparatus to prevent offsetting without interruption of high speed printing devices.